The most effective drug against extracellular mycobacteria is :

Ethambutol is a bacteriostatic drug. INH and rifampicin are equally effective against intra as well as extracellular mycobacteria. INH require a concentration of 0.25 µg/ml whereas rifampicin inhibits the growth of bacteria at a concentration of 0.05 µg/ml. Pyrazinamide acts more in acidic pH and it requires a concentration of 12.5 µg/ml. Thus, a most active drug for extra-cellular bacteria is rifampicin.